Surface modification of zinc oxide and electrophotographic member therefrom



y 1965 N. w, BLAKE ETAL 3,197,307,

SURFACE MODIFIGATIQN OF ZINC? QXIDE AND ELECTROPHQTOGRAPHIC MEMBERTHEREFRQM Filecf Sept. 22, 1964 '41 7 i i i g i 1 300 400* 50:0 600COATNG A A;

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a 3 4/: -,,,,//M,,,. 300 400 50W 6 3 com-m6 wAvsuENsrm 1M1 ACTIONSPECTRUM OF "WAWNGSENSI-HZEW WITH UNMODIFIED 2 nalll 9 /A "r:

' I A /A i A V V A z A %4 300 400 500 s 70o COATING E. ENG INA ACTIONSPECTRUM OF comma SENSITIZ wm-rmmm. VIOLEEMADE wrrn MODIFIED z cNollnanmlake rscnossmrcnso AREA am 40.; We g nnsn olscnmssnumsn INVENTORSUnited States Patent 3,197,367 SURFACE MGBKFICATKGN 6F ZHNC OXEDE ANDELECTRUPHGTOC-RAPHTC MER'EER THERE- FRQM Norman W. Blake, deceased, lateof Rochester, N.Y., by Bernice E. Blake, administratrix, Rochester,N.Y., and Cornelia C. Natale, Rochester, N.Y., assignors to EastmanKodak Company, Rochester, N.Y., a corporation of New Jersey Filed Sept.22, 1964, Ser. No. 460,63) 32 Ciairns. (Ci. %1)

This application is a continuation-in-part of application Serial No.75,753, filed December 14, 1960, now abancloned.

This invention is concerned with novel photoconduct-ive substancesformed by chemically modifying the surface of zinc oxide.Photoconductivity of zinc oxide is the basis of its use in a number ofuseful processes, such as photoconductography, xerography,photosensitive cells, and the like.

Photoconduc-tography is described in detail in British Patent 188,030,Von Bronk, and British Patent 464,112, Goldmann. British Patent 789,369,Berchtold, describes an improvement in the process using a protectivelayer against photoconductor and recording layers and Belgian Patent561,403, Johnson et al., describes, in considerable detail, systemsusing zinc oxide as a photoconductor.

Xerography is described in US. Patent 2,297,691, Carlson and others, Inone embodiment of the xerographic process, described in British Patent811,165, a photoconductive layer containing zinc oxide or a similarmaterial in a suitable resinous binder is provided with an electrostaticcharge and then exposed to a light image in a manner similar to thatemployed with ordinary silver halide photographic material. The chargeis dissipated in the areas where the light strikes the zinc oxide-resincoating. Subsequently, a colored toner powder is caused to adhere to thesurface by electrostatic attraction in those areas which have not beenexposed to the light and in which residual charge remains.

In the xerographic process, certain physical properties of the zincoxide coatings are important for successful operation. In particular,some of these properties are as follows:

(1) The relative effectiveness of different wavelengths of incidentlight in discharging the coating when a sensitizing dye is added to thezinc oxide dispersion.

(2) The relative efiiciency with which radiant energy absorbed by thezinc oxide discharges the coating, and

(3) The ability of the coating to hold an applied electrostatic chargein the dark.

These properties determine the utility of the coating. One (1) and two(2) taken together determine the photographic speed of the coating; thatis, they determine the amount of exposure to a given source of lightnecessary for image formation.

Three (3) determines whether or not the coating is useful in xerogra'phthe coating must retain the charge long enough in the dark to permitimage exposure and development.

The above three properties are considered in order below.

(1) This property is usually determined by measuring the action spectrumof a given coating. Unsensitized zinc oxide produces coating with actionspectra showing a maximum at about 390 millimicrons and falling rapidlyon either side so that light of 410 millimicrons and higher Seewavelengths produces little or no discharge. This dependence of suchcoatings upon light primarily in the ultraviolet part of the spectrumfor their photographic activity seriously limits their usefulness sincethey cannot be exposed with visible light. Since commonly used sourcessuch as tungsten light are much poorer in energy in the ultravioletrange than in the visible portion of the spectrum, exposure times ofthese coatings using such sources are relatively long.

It has been found that the action spectra of zinc oxide coatings may beextended by the addition of certain dyes to the zinc oxide dispersion.The resulting area of the visible spectrum encompassed by the actionspectrum of coatings incorporating dyes is found in practice to bedirectly related to the absorption spectrum of the dye used. However,not all dyes work equally well. In fact, with conventional zinc oxide,dyes fall clearly into two classes, those which sensitize appreciablyand those which sensitize only very slightly or not at all.

If a dye appreciably sensitizes a coating, then when the dye is presentin the coat-ing at a concentration of 1G mole/ gram of zinc oxide, theaction spectrum of the coating will show that light of wavelengthcorresponding to that most strongly absorbed :by the dye is at least 20%as effective as light absorbed directly by zinc oxide (at a wavelengthof 380 millirnicrons) in discharging the coating. Each of a very largenumber of dyes belonging to all common and generic classes has beencategorized according to this definition. It has been found previouslythat only those dyes which contain one or more of the following groupswill appreciably sensitize coatings based on conventional zinc oxide: acarboxylic acid group, an aromatic hydroxy group or an aromatic thiolgroup.

Studies of the partition of dyes between the zinc oxide surface and thebinder for the zinc oxide, show that only dyes containing the abovegroups absorb appreciably to the zinc oxide surface.

The limited number of effective dyes available includes certain dyes ofthe phthalein class, such as Rose Bengal and fluorescein, and certainsubstituted cyanine dyes. Even these dyes are not fully adsorbed to thezinc oxide. Moreover, each of these dyes encompasses only a relativelynarrow region of the spectrum. Therefore, these limitations haveprevented the full realization of the speed of the process when usingtungsten illumination, have required the use of multiple dyes to extendthe action spectrum throughout the visible (with concomitantdifiiculties in manufacture because of the varying solubilitycharacteristics of the different dyes) and have generally led to theproduction of highly colored coatings with relatively low-speed totungsten light sources.

(2) Changes in the relative efiiciency with which light energy absorbedby the zinc oxide discharges the coatings result in proportionatechanges in the speed of these coatings. Inability to change thisefiiciency has limited the applications of these coatings.

(3) It is valuable to be able to change the dark decay that is, the rateat which applied electrostatic charge is lost in the absence of light.Certain coatings have been unsuitable for customary use because of toorapid dark decay. These coatings have been those which contain a highconcentration of sensitizin dye. This increase in dark decay, as theconcentration of sensitizing dye is increased, critically limits thespeed that can be achieved through optical sensitization.

We have discovered new, modified zinc oxides which permit (1) theoptical sensitization of the zinc oxidecoat- 3 ing with nearly any dye,(2) changing the above described efiiciency of these coatings (eitherincreasing or decreasing it), and (3) changing the dark decay.

One object of this invention is to provide zinc oxide whose surface ischemically reacted with compounds to improve its use forelectrophotographic purposes. Another object is to provide a process formodifying zinc oxide to improve its electrophotographic properties. Anadditional object is to provide a chemically modified zinc oxide whichenables zinc oxide coatings to be sensitized with a great number ofdyes. Still another object is to provide chemically modified zinc oxidesallowing the preparation of coatings in which the above describedefficiency is changed. A further object is to provide chemicallymodified zinc oxides allowing the preparation of coatings in which thedark decay can be changed.

The above objects are obtained by chemically modifying zinc oxide of 98or higher percent chemical purity by attaching the following entities toa specified percent of its surface:

(a) One or more Lewis acids attached to to 100% of the surface of thezinc oxide. Typical Lewis acids include HCl, zinc chloride, acetylchloride, sulfur dioxide, sulfur trioxide, sulfuric acid, hydrogenbromide, boron fluoride, hydrogen bromide, acid bromides and chlorides,chlorosilanes, and the like. Lewis acids are' described in TheElectronic Theory of Acids and Bases, by W. F. Ludern and S. Zutfanti,John Wiley and Sons, Inc., New York, 1946, pages -17 and 43-46. It isunderstood,

of course, that these Lewis acids are not dyes.

Among Lewis acids which may be used are the following: hydrogen sulfide,hydrogen chloride, hydrogen fluoride, hydrogen bromide, sulfuric acid,phosphoric acid, sulfur dioxide, sulfur trioxide, aluminum chloride,

boron fluoride, zinc chloride, zinc bromide, thionyl chloride, acetylchloride, acetyl bromide, trifluoroacetic acid, iodoacetic acid,thioglycolic acid, benzoyl chloride, adipyl chloride, adipyl bromide,terephthaloyl chloride, isophthaloyl chloride, maleic anhydride, silicontetrachloride, trichloromethylsilane, dichlorodimethylsilane,chlorotrimethylsilane, trichlorophenylsilane, dichlorodiphenylsilane,chlorotriphenylsilane, chlorodimethylphenylsilane andchloromethyldiphenylsilane. All other Lewis acids, which are not dyes,are operative and may be used. Particularly useful Lewis acids are thosewhich do not absorb light having a wavelength between 400 and 700 my (b)One or more of the salts of aluminum, bismuth,

chromium, copper, iron, lithium, magnesium, nickel, tin and titaniumattached to 10 to 100 percent of the zinc oxide surface, and

(c) Any of a number of ditferent reducing agents such as tannic acid,gentisic acid, pyrogallol, benzylaminophenol, ascorbic acid, stannoussalts, and the like, having in common that they are at least as powerfulreducing agents as stannous ion, attached to 0.1 to percent of the zincoxide surface- Compositions provided in group (a) are useful in thepreparation of electrophotographic coatings, for instance, xerographiccoatings. Coatings prepared from them show greatly increasedeffectiveness of optical sensitization by dye surmounting thelimitations mentioned in (1) above. Coatings from them show doubling ofthe efiiciency, defined in (2) above. They also show much decreased'rate of dark decay of the applied electrostatic charge,

particularly at high concentrations of dye, overcoming the limitationsdescribed in (3) above.

Compositions provided in group (b) are useful in the preparation ofxerographic coatings. For instance, coatings may be prepared from thesecompositions for which the efficiency, defined in (2) above, isdecreased by a factor of 3 or more.

Compositions provided in group (c) are useful in the preparation ofxerographic coatings. For instance, through their use coatings havinggreatly increased rates of dark decay of applied electrostatic chargemay be prepared.

The majority of modifiers fall into a single category, (a), (b), or (c).When a particular modifier falls into category (c) and any of theothers, the effect obtained from compositions provided in group (c)predominates. When a modifier falls into (a) and (b), the effectobtained from compositions of group (b) predominates.

In our preferred embodiment, zinc oxide of 98 or higher percent chemicalpurity is slurried into a chemically inert solvent, such as toluene, thefinal composition being between 5 and percent zinc oxide. Sufficientmodifying chemicals to react with the desired percentage of the surfaceof the zinc oxide is dissolved separately in a relatively small quantityof solvent such as acetonitrile, dimethylformamide, methanol or toluene.The zinc oxide slurry is agitated violently, such as through the use ofa Waring Blendor, while the modifier solution is added dropwise over aperiod of minutes. After addition of the modifier is complete, otheraddenda, such as polymer(s) and dye(s) may be added to the slurry, andcoatings prepared. Another preferred method is useful when the modifiercan be vaporized, for instance, when using a modifier such as hydrogenchloride or (CH SiCl. In this method, dry zinc oxide powder is stirredrapidly in an enclosed vessel, to which the modifier vapor is slowlyadded over a period of minutes. Vaporization can be accelerated byheating or by reducing pressure, or by a combination of these two steps.Saturating a stream of inert gas with vapor of the treating chemical andcausing it to fiow in contact with the zinc oxide is also a means ofaccomplishing the desired effect. The resulting modified Zinc oxide maythen be used to prepare electrophotographic coatings in any of theconventional ways.

In many instances, the modifier reacts with the zinc oxide vigorously,and may react with more than just the surface. In the event that thereaction is not controlled carefully, the zinc oxide may become amixture of unmodified and overrnodified zinc oxide which would not be asuseful for electrophotographic purposes. The unmodified and theoverrnodified would both have inferior properties to that of theproperly modified zinc oxide. Accordingly, highly reactive modifierssuch as hydrogen chloride and acid chlorides must be added to the zincoxide so that equal exposure of each of the zinc oxide particles to themodifier solution occurs. This is accomplished by continuously andrapidly agitating the zinc oxide with the addition of the modifier at alow rate of addition.

The amount of modifier necessary to react with the desired percentage ofthe surface depends upon the area of the surface of the particular zincoxide being treated.

Surface area data may be obtained from the manufacturer of the zincoxide or the surface area may be measured by the known method ofnitrogen adsorption.

In order to compare the properties obtained using zinc oxide modifiedaccording to our invention, practical methods have been used to measurethe properties described above as (1), (2) and (3).

(1) The relative effect of various wavelengths of incident radiation indischarging a zinc oxide coating is determined by exposing a chargedcoating of dimensions X by Y to a spectrum of radiation, said spectrumchanging wavelengths along the X direction and being subject to stepwiseintensity attenuation along the Y direction (log of exposure) at eachwavelength. Subsequent toning of the sample defines a curve, thespectral response curve of the sample, indicating the effectiveness ofvarious wavelengths of light in discharging the sample. This curve iscalled the action spectrum of the sample. The spectral distribution ofenergy of the source producing the spectrum will affect the shape ofthis curve. However, in practice, a known source, a tungsten filamentlamp operated at 3000 K., so that its spectral distribution remainsconstant, is used. This permits comparison between various actionspectra.

Conventional, undyed zinc oxide produces coatings with action spectra asshown in FIGURE 1, Coating A, of the attached drawing, showing a maximumat about 380 millimicrons and falling rapidly on either side, so thatlight of 410 millimicrons and higher wavelengths produces little or nodischarge. FIGURE 2, Coating B, shows the action spectrum of a coatingmade with unmodified zinc oxide sensitized with Crystal Violet. FIGURE3, Coating B, shows the action spectrum of a coating made with modifiedzinc oxide sensitized with Crystal Violet. In each instance, 5x10" moleof Crystal Violet per gram of zinc oxide was used for the sensitizingagent. The zinc oxide was modified as described in Example 1, using amodifier from group (a) above.

(2) The relative efficiency of radiant energy absorbed by the zinc oxidein discharging zinc oxide coatings is derived from measurements of therelative speeds of the coatings. The speed of a coating depends upon twofactors: How much of the incident radiant energy the coating absorbs andhow eiiiciently the absorbed radiation is used to discharge the coating.Intercomparison of the speed values of two different coatings havingidentical absorption spectra, determines the efficiency of onewithrespect to the other.

Changes in the relative efficiency of absorbed light energy indischarging the coatings will result in propor tionate changes in thespeeds of the coatings. For certain applications (e.g., exposure withilluminants of low intensity), it is necessary to have the speed asgreat as possible and increases in the relative efiiciency of theprocess are desirable. In other applications, very low speed is needed(e.g., for handling in room light) and decreases in the relativeefficiency of the process are wanted. Thus, the ability to change thisefliciency through the use of modified zinc oxide extends the range ofapplication of zinc oxide coatings.

The speed of xerographic coatings is measured as follows:

A coating is charged in the dark and mounted in a sensitometer,immediately behind a transparency of step- Wise-increasing neutraldensity. Each step in this transparency attenuates the light reachingthe surface of the coating by approximately 25 percent more than thepreceding step. The most transparent step allows 19 footcandles ofillumination, from a 3000 K. tungsten source, to strixe the sample. Thesample is exposed in the sensitometer for 3 seconds and toned in astandard fashion. The first step of the toned sample to which no toneris held indicates the speed of the sample. Since speed is relative, thespeed number associated with the most transparent step of the scale isarbitrarily placed at 20. Succeeding steps have speed numbers of 25, 31,39, 49, 61, and so forth. 'The higher the number, the faster the speed,and the more dense the step; that is, the less light used to dischargethe sample. On this scale, unsensitized zinc oxide coatings, prepared ina standard fashion, have a speed of approximately 60. Accuracy is withinabout 25 percent or one step.

(3) The ability of zinc oxide coatings to hold an applied electrostaticcharge in the dark is determined by charging a sample and observing itssurface potential, as a function of time, in the dark. Measurement isstarted immediately after charging and continued for 3 minutes. Thepotential immediately after charging and at /2, 1, 2 and 3 minutes aftercharging is noted. Frequently, the time necessary for the initialpotential to drop to half of its value is also noted. 7

It is desirable to be able to change the rate at which appliedelectrostatic charge is lost in the absence of light. In someapplications it is important to reduce this rate. Thus, xerographicutility of zinc oxide coatings requires that they hold an appreciablefraction of an applied charge in the dark for a period of time equal tothat necessary to expose and tone such coatings. If longer than 3minutes is required for the initial potential to drop to halfvalue,charge retention of the coating is adequate.

Unsensitized conventional zinc oxide coatings require longer than 3minutes for the initial potential to drop to half-value, when theapplied electrostatic charge is nega-' tive. 'If the appliedelectrostatic charge is positive, the potential of the surface drops tohalf-value in less than 15 seconds. Thus, such coatings are difiicult orimpossible to use with positive charging even though this polarity ofcharging is desirable in certain applications. With dye-sensitizedcoatings, it has been observed that the greater the concentration of dyein the coating the more rapid the dark decay of charge. At very high dyelevels, even with negative charging, the dark decay is frequently sorapid the coating is useless for xerographic purposes. The data in TableV illustrates this. The increase in dark decay as the dye concentrationis increased thus effectively limits the speed achievable throughoptical sensitization, as it limits the amount of dye that can be used.

In the practice of this invention, the particle size of the zinc oxidewhich may be used may be from 0.01 micron to 5.0 microns. Surfacemodification does not require zinc oxide of any particular size; thesize requirement of zinc oxide particles depends upon their ability tobe dispersed in a given binder for electrophotographic purposes. Forinstance, any zinc oxide of a size which can be used forelectrophotographic purposes can be surface modified Within the scope ofour invention, to obtain the improved characteristics described herein.

In order to provide a coating for electrophotographic purposes, modifiedzinc oxide may be dispersed in a polymeric coating of various types in aratio of 0.5 to 6 parts of modified zinc oxide to about one part of anorganic polymeric binder such as a cellulose ester, polymers derivedfrom styrene and butadiene, polystyrene, polyvinyl chloride,polyvinylacetals, poly-n-butylmethacrylate, polyolefins, polyesters,polyamides, and the like. The only requirement for the binder forxerographic use is that coatings of it, free from zinc oxide, be capableof holding an applied electrostatic charge. Suitable coatings have adielectric constant of about 9-2.5. These coatings may contain 0.1 X 10-to 50 X 10- mole of dye per gram of zinc oxide. Any type of dye may beused.

The following examples are intended to describe our invention but arenot intended to limit it in any Way.

EXAMPLE 1 168 grams of zinc oxide were added to 238 grams of xylene in awater-jacketed Waring Blendor. The particle size of the zinc oxide mostfrequently occurring was 0.1 micron and of the particles were less than0.4 micron. Sixty milliliters of xylene, in which the Lewis acid S0 wasdissolved, were then added dropwise, with thorough mixing. The slurrywas then stirred for 10 minutes, at the end of which time 8.4 10- moleof Crystal Violet dissolved in 15 grams of methanol was added. Themixture was stirred for 5 minutes, then 14.4 grams of a 60% solidssolution in toluene of an organopolysiloxane resin was added, followedby addition of 111.9 grams of a 30% sohds solution in toluene of astyrenebutadiene polymer. The mixture was then stirred for 3 minutes. Atthe end of this time, it was coated on paper to give a coverage of 3grams of dry coating per square foot. Similar coatings were made, asshown below, Without modification with the Lewis acid and without thedye. These coatings served as controls. The coatings were conditionedfor three days at 5 0% relative humidity at 70 P. Then the actionspectra and speed were determined as described above. These coatings aredescribed and the speed data obtained from them are given in Table I.

e 8 r Table I S In each case, an increase in speed of a factor of 2 wasobserved.

SO Coating a Dye ires- Speed Table H en r Moles Gram THE OPTIMUM AMOUNTSOF VARIOUS news news NEEDED TO OPTIMUMLY MODIFY ZINC OXIDE 0 60 4 gLewis acid: Moles acid/ 168 g. ZnO demo- 2,40% 0 I101 1 4. 10-: Acetylch oride 4. 10- $335582; Adipyl chloride 2.25 10 g Cyanuric chloride225x10- $19383 Phenyl trichlorosilane 1.5 Dimethyl dichlorosilane225x10- Trimethyl chlorosilane 4.5 1() Action spectra of Coatings A, B,and E are given 1n HF FIGURES 2 9 Zinc chloride 2.25 10 Optimumsensitization of the dye occurs when S03 2 25X1O 3 2.25 10 mole of S0are used for 168 grams of Zinc sulfamic acid oxide. Based on thereaction ZnO+SO ZnSO this corresponds to approximately percent of thesurface zinc oxide reacted for the sample of zinc oxide used. Thus, themodification of zinc oxide with the optimum amount of S0 doubles thespeed of the coating, even in the absence of dye. Therefore, theefliciency of the coating is doubled. Reaction of the zinc oxide with S0was determined by tests of the supernatant liquid obtained fromcentrifuging the slurry after addition of the S0 but prior to theaddition of the dye. No S0 was found to be present.

EXAMPLE 2 The procedure used to prepare coatings C to I of Example l wasused to prepare a series of coatings from each of the Lewis acids inTable II in place of S0 Within each series the amount of modifier usedvaried from 03x10 to l l0 mole. In those cases where the Lewis acid wasnot sufiiciently soluble in xylene, it was dissolved in another suitablesolvent such as methanol. The speeds of these coatings were thendetermined as described above. The amount of modifier with which maximumspeed was obtained is given in Table II and is called the optimum amountof modifier.

Coatings were then prepared using the procedure used to prepare coatingJ of Example 1, but using the abovedetermined optimum amount of Lewisacid in place of In each instance, reaction occurred between themodifier and zinc oxide. No modifier could be detected in thesupernatant liquid obtained from centrifuging the slurry after additionof the modifier but prior to the addition of dye.

EXAMPLE 3 The procedure of Example 1 with 4.5 l0 mole of hydrogenchloride as modifier, replacing the S0 of Example 1, and 5x10 mole ofdye per gram of zinc oxide was used to prepare coatings from the dyeslisted below in Table III. Coatings were also prepared using unmodifiedzinc oxide. In certain cases the percent of the added dye adsorbed tothe zinc oxide was determined, by measuring the amount of dye present inthe supernatant liquid produced by centrifuging the slurry. Actionspectra of the coatings prepared from modified zinc oxide showedsensitivity in the Wavelength region of light absorbed by the dye, inaccordance with increased speeds as shown in Table III. Action spectraof the coatings prepared from unmodified zinc oxide showed little or nosensitivity in the spectral range of light absorbed by the dye, exceptfor the blue-red cyanine dye and the dyes fluorescein and Rose Bengal,each of which has either a COOH or an aromatic OH group attached to it.

Table III OPTICAL SENSITIZAIION OF ELECTROPHOTOGRAPHIC COATINGS BASED ONEITHER MODIFIED OR UNMODIFIED ZINC OXIDE Modified Z110 Unmodified ZnODye Type Percent Speed Percent Speed dye on dyo on ZnO Z110 Blue-red cyine Cy'min 99 4, 250 2. 030 Quinaldine Red 0 90 2, G20 20 204 MalachiteGreen... Triphenylmethaue 60 995 67 Lithosol Blue 6G do 2, 620 3 CrystalViolet 4. 250 131 Amanil Black ROL... 164 43 p-Methyl Red 250 23IOutacyl Blue-Black S3 204 67 Nigrnsinp 0) 204 17 Chrysoidine 4 164 43Methylene Blue 4, 250 104 Chrome Blue GCB 500 131 Phenosafranin 90 995131 1, 4-diaminoanthraquinone 25 250 83 D & C Violet #2 base 164 23Alizarin 50 315 204 Celanthreue Red 0) 164 55 fluorescein"- 7 91 50 315Rliodnmine B 3. 380 500 Ross Bengal 90 995 60 627 Phosphine RN. Acridiue500 104 Acridine Orange do 627 83 Naphthol Yellow 8.. Nitro 104 43Auramine O Diphenyl methane..- 627 55 Thiofiavine TG 'lhiazolc 500 67 2Little or no dye adsorbed.

9 EXAMPLE 4 The procedure of Example 1 was used to prepare the coatingsof Table lV below replacing the S of Example 1 with the modifier listedin Table IV. Dye Was not added to any of the coatings of Table IV inorder that speed measurements could be used as efirciency measurements.In those instances where the modifier of Table IV was not entirelysoluble in xylene, a solvent such as methanol, acetonitrile, ordimethylformamide was substituted.

The data in Table IV show that modification for these compoundsdecreases the efliciency with which radiation, absorbed by the zincoxide, causes discharge of the coatin EXAMELE 224 grams of zinc oxidewere sealed in a vessel provided with a stirrer capable of tumbling thedry powder over upon itself rapidly. 200 ml. of gaseous hydrogenchloride at atmospheric pressure and at 26 C. were introduced slowlyinto the vessel. After addition of the hydrogen chloride, a solution of3l7.1 grams of toluene, 20.9 grams of methanol and 0.03 gram of CrystalViolet dissolved in about it ml. of methanol were ad ed. The resultingmixture was dispersed in a high shear mixer for minutes. Twelve grams ofa 69 percent solids solution in toluene of the organopolysiloxane resinwas added and dispersed for one more minute. Finally, 96 grams or" a 30percent solids solution in toluene of a styrene-butadiene copolymer wasadded and mixed for another 2 minutes. The dispersion was then coated ona paper support and the resulting xerographic printing paper had a speedof 5300. In other identical tests, the speeds or" the coatings rangedfrom 3400 to 5300.

EXAMPLE 5 Zinc oxide was treated with hydrogen chloride, as described inExample 5. One portion of the zinc oxide was immediately blended withsolvent, polymers and dye and coated as described. The resulting speedof the coating was 2606.

After three days a second portion of the modified zinc oxide wascompounded and coated in the same manner. The speed or" this coating wasagain 2600.

After seven days, the third portion of the modified zinc oxide wascompounded and coated in the same manner. The s, ecd or" this coatinrwas 200%. This value is only approximately less than the 2600 obtainedand is within the experimental limits of the general results obtained inthe process.

EXAMPLE 7 4.5 x 0* moles of acetyl face-modified using 2,364 ml. ofgaseous HCl.

16 oxide gives increased speed and the dark decay is not adverselyaffected at high dye levels.

Table V EFFECT OF SENSITIZATION LEVEL ON DARK DECAY OF ELECTROSTATICCHARGE USING COATINGS BASED ON (a) UNMODIFIED AND (b) MODIFIED ZnO Molesoi blue-red cyanine Speed V V V Tv/z dye/g. of Z110 (2.) 5X10- 580 470310 355 325 215 600 480 2% min. 420 390 340 370 220 min. 300 270 180 Vis the initial potential (in volts) oi the applied electrostatic charge71, is the potential (in volts) of the applied electrostatic charge,after 36 minute in the dark.

V; is the potential of the applied electrostatic charge, after decayedfor 3 minutes in the dark.

Tv/2 is the time (in minutes) necessary for the applied electrostaticcharge to decay to half of its initial potential.

EXAMPLE 8 168 grams of zinc oxide were added to 238 grams of commercialxylene in a water-jacketed quart Waring Blendor. jar, and 60 ml. ofxylene containing 45x10- equivalents of hydrogen chloride were addedslowly with thorough mixing. The slurry was mixed for 10 minutes, at theend of which time 5.1 grams of methanol containing 6.02345 gram ofp-rnethyl red and 6.0137 gram of Crystal Violet were added and themixture stirred for 5 minutes. Then, 14.4 grams of an organopolysiloxanesolution, 60 percent solids in toluene, was added, and the mixturestirred for one minute. it the end of this time 111.9 grams or" astyrene-butadiee copolymer, percentsolids in toluene, was added and themixture stirred for 2 minutes. The mixture was then coated on paper anddried. its speed is 2600.

The above procedure, omitting the hydrogen chloride addition, yields acoating with a speed of 100 or less.

EXAMPLE 9 2017 grams of toluene and grams of methanol were mixed in agallon Waring Blender. 2657.4 grams of zinc oxide were added over a-minute period. The blender was water-iaclreted at 166 F. (a) 154 gramsof this slurry were transferred to a pint Waring Blender; 4 grams ofmethanol were added followed by 54 grams of a 33 /3 percent solution ofthe resins comprising 88 parts of styrene-outadiene copolymer solution,percent solids in toluene and 10 parts of organopolysiloxane solution,3-1) percent solids in toluene, and 10 parts of Piccopale hydrocarbonresins. The slurry was stirred 5 minutes and then coated onto aluminumfoil. (b) The remainder of the zinc oxide slurry in the gallon WaringBlender was sur- 154 grams of this slurry of surface-modified zinc oxidewere then treated as in (a) above.

A sample of each of the two coatings thus made was exposed for 10seconds, to 400 foot candles of tungsten illumination incident upon aphotographic step tablet in contact with the photoconductive surface;The conductivity pattern induced by this exposure was then developed toa visible image by an essentially two-step process (the firstelectrolytic, the second chemical) in the following way:

A cellulose sponge saturated with an aqueous solution of 1.0% manganousnitrate was drawn several times across the surface of the exposedphotoconductographic surface, the sponge being held at a 60-voltpositive potential with respect to the aluminum foil backing of thephotoconductive layer. The layer surface was then dried With anabsorbent tissue. The image material, deposited by this electrolyticstep was then rendered visible by treatment with a 5% by weightsilver-nitrate solution, after which the print surface was rinsed withwater and blotted dry.

1 1 The photoconductographic paper containing the modified zinc oxidewas two to four times faster in photographic speed than the papercontaining unmodified zinc oxide.

EXAMPLE 10 Use of a base, e.g. potassium hydroxide: 56 grams of zincoxide were treated with 2 l0 moles of KOH in methanol. The zinc oxide sotreated, in a binder, was coated out. The coating had a V of 30 voltsand a 7- of 30 seconds. A similar coating from zinc oxide not treatedwith KOH had a V 0 of 500 volts and'a of 3 minutes.

EXAMPLE 11 of 45 seconds. In the absence of treatment withhexamethylenediamine, a similar coating had a speed of 1252, V of 550volts and a g of 3 minutes.

Photoconductive zinc oxide referred to herein is intended to be zincoxide which may be prepared by various methods including the Frenchprocess, the wet process, the American process and the like, provided itis at least 98% pure. Typical methods of preparing zinc oxide arereferred to in A Volume-Charge Capacitor Model for Electrofax Layers, J.A. Amick, RCA Review, vol. 20, 770-784, December 1959. The particularmethod of preparing the zinc oxide is not limiting to our inventionproviding the zinc oxide is hotoconductive in nature.

The units expressed herein have been based on grams of zinc oxide forconvenience. It will be appreciated that the range of proportions can beexpressed as 014x10- to 7.2)(' mole of modifier per mole of zinc oxideor 1.8x 10- to 9.() 10* mole of modifier per gram of zinc oxide.

By conducting substrates are intended electrically conducting glass,zinc, aluminum and similar metallic substances, resinous materialshaving conductors incorporated therein to render them conductive,polymeric. materials having metallic coatings thereon, and the like.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

We claim:

1. Finely divided hotoconductive zinc oxide of improvide dye adsorptioncontaining on at least 10% of the surface of the zinc oxide particles, amodifier in the concentration of 0.14 10 to 7.2 10- mole of modifier permole of zinc oxide, obtained by reacting the zinc oxide with a Lewisacid stronger than zinc oxide.

2. Finely divided hotoconductive zinc oxide .of improved dye absorptioncontaining on at least 10% of the surface of the zinc oxide particles, amodifier in the concentration of 014x10 to 7.2 l0 mole of modifier permole of zinc oxide, obtained by reacting the zinc oxide with aluminumchloride.

3. Finely divided photoconductive zinc oxide of improved dye adsorptioncontaining on at least 10% of the surface of the Zinc oxide particlse, amodifier in the concentration of 0.14 10* to 7.2 1() mole of modifierper mole of zinc oxide, obtained by reacting the zinc oxide 4. Finelydivided hotoconductive zinc oxide of improved dye adsorption containingon at least 10% of the surface of the zinc oxide particles, a modifierin the con centration of 0.14 10 to 72x10 mole of modifier per mole ofZinc oxide, obtained by reacting the zinc oxide with HCl.

5. Finely divided hotoconductive zinc oxide of improved dye adsorptioncontaining on at least 10% of the surface of the zinc oxide particles, amodifier in the concentration of 0.l4 l0" to 7.2)(10 mole of modifierper mole of zinc oxide, obtained by reacting the Zinc oxide with acetylchloride.

6. Finely divided photoconductive Zinc oxide of improved dye adsorptioncontaining on at least 10% of the surface of the zinc oxide particles, amodifier in the concentration of 0.l4 10 to 72x10 mole of modifier permole of zinc oxide, obtained by reacting the zinc oxide withhexamethylenediamine.

7. A photographic element for use in electrostatic photographicprocesses comprising 3090% of a photoconductive finely divided zincoxide of improved dye adsorption containing on at least 10% of thesurface of the zinc oxide particles, a modifier in the concentration of().14 10 to 72x10" mole of modifier per mole of zinc oxide, obtained byreacting the zinc oxide with a Lewis acid stronger than zinc oxide, saidzinc oxide dispersed in an insulating film-forming medium and coated ona non-conducting substance.

8. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of photoconductive zinc oxide containing onat least 10% of the surface of the zinc oxide of improved .dyeadsorption particles, a modifier in the concentration of 0.l4 10' to 7.210 mole of modifier per mole of zinc oxide, obtained by reacting thezinc oxide with aluminum chloride, dispersed in an insulatingfilm-forming medium and coated on a non-conducting substrate.

9. A photographic element for use in electrostatic photographicprocesses comprising 3090% of photoconductive zinc oxide containing onat least 10% of the surface of the zinc oxide of improved dye adsorptionparticles, a modifier in the concentration of 0.14 10* to 7.2 10- moleof modifier per mole of zinc oxide, obtained by reacting the zinc oxidewith S0 dispersed in an insulating film-forming medium and coated on anonconducting substrate.

10. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of photoconductive zinc oxide containing onat least 10% of the surface of the zinc oxide of improved dye adsorptionparticles, a modifier in the concentration of 0.l4 10- to 7.2 10 mole ofmodifier per mole of zinc oxide, obtained by reacting the zinc oxidewith HCl, dispersed in an insulating film-forming medium and coated on anon-conducting substrate.

11. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of photoconductive zinc oxide containing onat least 10% of the surface of the zinc oxide of improved dye adsorptionparticles, a modifier in the concentration of 0.14 l0 to 7.2X10- mole ofmodifier per mole of zinc oxide, obtained by reacting the zinc oxidewith acetyl chloride, dispersed in an insulating film-forming medium andcoated on a non-conducting substrate.

12. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of photoconductive zinc oxide containing onat least 10% of the surface of the zinc oxide of improved dye adsorptionparticles, a modifier in the concentration of 0.l4 10- to i3 7.2 10-mole of modifier per mole of zinc oxide, obtained by reacting the zincoxide with hexamethylenediamine, dispersed in an insulating film-formingmedium and coated on a non-conducting substrate.

13. A photographic element for use in photoconductographic processescomprising 30-90% of a finely divided photoconductive zinc oxide ofimproved dye adsorption containing on at least 10% of the surface of thezinc oxide particles, a modifier in the concentration of 0.14 10 to7.2x10 mole of modifier per mole of zinc oxide, obtained by reacting thezinc oxide with a Lewis acid stronger than zinc oxide, dispersed in aninsulating film-forming medium, and coated on a conducting substrate.

14. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of a finely divided photo'conductive zincoxide of improved dye adsorption containing on at least 10% of thesurface of the zinc oxide particles, a modifier in the concentration of0.14 10 to 7.2 10 mole of modifier per mole of zinc oxide, obtained byreacting the zinc oxide with aluminum chloride, dispersed in aninsulating film-forming medium and coated on a conducting substrate.

15. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of a finely divided photoconductive zincoxide of improved dye adsorption containing on at least 10% of thesurface of the zinc oxide particles, at modifier in the concentration of014x10" to 7.2 10 mole of modifier per mole of zinc oxide, obtained byreacting the zinc oxide with S dispersed in an insulating film-formingmedium and coated on a conducting substrate. 7

16. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of a finely divided photoconductive zincoxide of improved dye adsorption containing on at least 10% of thesurface of the zince oxide particles, a modifier in theconcentration of0.14X10 to 7.2 mole of modifier per mole of zinc oxide, obtained byreacting the zinc oxide with HCl, dispersed in an insulating film-forming medium and coated on a conducting substrate.

17. A photographic element for use in electrostatic photographicprocesses comprising -90% of a finely divided photoconductive zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe zinc oxide particles, a modifier in the concentration of 014x10- to72x10 mole of modifier per mole of zinc oxide, obtained by reacting thezinc oxide with acetyl chloride, dispersed in an insulating film-formingmedium and coated on a conducting substrate.

18. A photographic element for use in electrostatic photographicprocesses comprising 30-90% of a finely divided photoconductive zincoxide of improved dye adsorption containing on at least 10% of thesurface of the zinc oxide particles, a modifier in the concentration of0.14 10 to 7.2 10- mole of modifier per mole of zinc oxide, obtained byreacting the zinc oxide with hexamethylenediamine, dispersed in aninsulating film-forming medium and coated on a conducting substrate.

19. A photographic element for use in an electrostatic photographicprocess comprising 30-90% of a finely divided photoconductive zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe zinc oxide 014x10 to 7.2 l0 mole of modifier per mole of zinc oxide,containing a dye adsorbed to the zinc oxide, dispersed in an insulatingfilm-forming medium and coated on a non-conducting substrate, saidmodifier obtained by reacting zinc oxide with a Lewis acid stronger thanzinc oxide.

20. A photographic element for use in an electrostatic photographicprocess comprising 30-90% of a finely divided photoconductive zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe zinc oxide 0.14 l0 to 7.2 10 mole of modifier per mole of zincoxide, containing a dye adsorbed to the id modified zinc oxide,dispersed in an insulating film-forming medium and coated on anon-conducting substrate, said modifier obtained by reacting zinc oxidewith aluminum chloride.

21. A photographic element for use in an electrostatic photographicprocess comprising 30-90% of a finely divided photoconductive zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe zinc oxide 0.14 l0 to 7.2 10* mole of modifier per mole of zincoxide, containing a dye adsorbed to the modified zinc oxide, dispersedin an insulating filmforming medium and coated on a non-conductingsubstrate, said modifier obtained by reacting zinc oxide with 22. Aphotographic element for use in an electrostatic photographic processcomprising 30-90% of a finely divided photoconductive zinc oxide ofimproved dye adsorption containing on at least 10% of the surface of thezinc oxide 0.14 10- to 7.2)(10 mole of modifier per mole of zinc oxide,containing a dye adsorbed to the modified zinc oxide, dispersed in aninsulating film-forming medium and coated on a non-conducting substrate,said modifier obtained by reacting zinc oxide with HCl.

23. A photographic element for use in an electrostatic photographicprocess comprising 3090% of a finely divided photoconductive Zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe Zinc oxide 0.14 1O to 7.2x 10" mole of modifier per mole of zincoxide, containing a dye adsorbed to the modified zinc oxide, dispersedin an insulating film-forming medium and coated on a non-conductingsubstrate, said modifier obtained by reacting zinc oxide with acetylchloride.

24. A photographic element for use in an electrostatic photographicprocess comprising 30-90% of a finely divided photoconductive zinc oxideof improved dye adsorption containing on at least 10% of the surface ofthe zinc oxide 0.14 10 to 72x10 mole of modifier per mole of zinc oxide,containing a dye adsorbed to the modified zinc oxide, dispersed in aninsulating film-forming medium and coated on a non-conducting substrate,said modifier obtained by reacting zinc oxide with hexamethylenediamine.

25. A process for improving the dye adsorption of photoconduct-ive zincoxide comprising contacting finely divided zinc oxide with a Lewis acidstronger than zinc oxide, sutficiently to incorporate on at least 10% ofthe surface of the zinc oxide 0.14 10 to 72x10 mole of modified zincoxide per mole of zinc oxide.

26. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the finely divided zinc oxide with a gaseousLewis acid in order to incorporate on at least 10% of the surface of thezinc oxide O.14 10 to 7.2 10 mole of modified zinc oxide per mole ofzinc oxide, obtained by reacting zinc oxide with the gaseous Lewis acidstronger than zinc oxide.

27. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the finely divided photoconductive zincoxide With a surface modifying material in order to incorporate on thesurface of the Zinc oxide from 0.14 10 to 7.2 10- mole of modified zincoxide per mole of zinc oxide and then adding dye, said modified zincoxide determined by reacting the zinc oxide with a Lewis acid strongerthan zinc oxide.

23. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the finely divided photoconductive zincoxide with a surface modifying material in order to incorporate on thesurface of the zinc oxide from 0.14 10- to 72x10" mole of modified zincoxide per mole of zinc oxide and then addind dye, said modified zincoxide obtained by reacting the zinc oxide with aluminum chloride.

29. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the finely divided photoconductive zincoxide with a surface modifying material in order to incorporate on thesurface of the zinc oxide from O.14 10" to 7.2)(10 mole of modified zincoxide per mole of zinc oxide and then adding dye, said modified zincoxide obtained by reacting the zinc oxide with S0 30. A process forimproving the dye adsorption of photoconductive zinc oxide comprisingcontacting the finely divided photoconductive zinc oxide with a surfacemodifying material in order to incorporate on the surface of the zincoxide from 0.14 10 to 7.2 1()- mole of modified zinc oxide per mole ofzinc oxide and then adding dye, said modified zinc oxide obtained byreacting the zinc oxide With HCl.

31. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the 16 finely divided photoconductive zincoxide with a surface modifying material in order to incorporate on thesurface of the Zinc oxide from 0.14 10- to 7.2 10" mole of modified zincoxide per mole of zinc oxide and then adding dye, said modified zincoxide obtained by reacting the zinc oxide with acetyl chloride.

32. A process for improving the dye adsorption of photoconductive zincoxide comprising contacting the finely divided photoconductive zincoxide with a surface modifying material in order to incorporate on thesurface of the zinc oxide from 0.14 10* to 72X 10- mole of modified zincoxide per mole of zinc oxide and then adding dye, said modified zincoxide obtained by reacting the zinc oxide with hexamethylenediamine.

No references cited.

NORMAN G. TORCHIN, Primary Examiner.

7. A PHOTOGRAPHIC ELEMENT FOR USE IN ELECTROSTATIC PHOTOGRAPHIC PROCESSES COMPRISING 30-90% OF A PHOTOCONDUCTIVE FINELY DIVIDED ZINC OXIDE OF IMPROVED DYE ADSORPTION CONTAINING ON AT LEAST 10% OF THE SURFACE OF THE ZINC OXIDE PARTICLES, A MODIFIER IN THE CONCENTRATION OF 0.14X10-5 TO 7.2X10-3 MOLE OF MODIFIER PER MOLE OF ZINC OXIDE, OBTAINED BY REACTING THE ZINC OXIDE WITH A LEWIS ACID STRONER THAN ZINC OXIDE, SAID ZINC OXIDE DISPERSED IN AN INSULATING FILM-FORMING MEDIUM AND COATED ON A NON-CONDUCTING SUBSTANCE. 